Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976. The Grants found changes from one generation to the next in beak shape distribution with the medium ground finch on the Galápagos island of Daphne Major. The birds have inherited a variation in their bill shape with some having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Niño, there was a lack of large hard seeds which the large-billed birds ate; however, there was an abundance of the small soft seeds which the small-billed birds ate. In the years following this El Niño, the Grants measured beak sizes in the population and found that the average bill size was smaller. As conditions improved in 1987

Question

and larger seeds became more available, the trend toward smaller average bill size ceased. Why were these measurements important?

a. They showed that birds with different bill sizes eat different types of food and therefore do not compete with one another for resources.
b. They revealed that weather can have a strong effect on the food preferences of the birds.
c. They provided strong evidence of evolution by natural selection because birds with small bills had more offspring in years where small seeds were prevalent, and large-billed birds were more likely to survive and breed in years with large seeds.
d. They showed that bill size changes in response to the available food supply. This was a unique adaptation not formerly seen in any species of bird.

Answer 1

c. They provided strong evidence of evolution by natural selection because birds with small bills had more offspring in years where small seeds were prevalent, and large-billed birds were more likely to survive and breed in years with large seeds.